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2182A Integration Time

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luiz.andre
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Joined: February 26th, 2013, 5:41 am
Country: Brazil

2182A Integration Time

Post by luiz.andre » February 26th, 2013, 5:53 am

Fellow companions,

I've been doing nanovolt measurements on superconducting tapes using the 2182A nanovoltmeter. I need fast measures because if the current passes through the superconductor for a long period of time, it may damage it. So I've been using NPLC 1, for the low noise and supposedly 16ms integration time, which would suit me well, but when it takes a measure, the command READ? takes about 70ms to give me a result! What is wrong here? NPLC 5 also takes a LOT longer than the expected 80ms.

I thank you very much in advance for the help!

Dale C
Keithley Applications
Keithley Applications
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Joined: June 10th, 2010, 6:22 am
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Re: 2182A Integration Time

Post by Dale C » February 26th, 2013, 7:18 am

The NPLC is only one part of the complete A/D conversion.
That is why the reading rate is not 16 msec.
The A/D Conversion has four parts.
1. Signal Integrate - This is the time the unit "looks" at the input signal.
2. Zero Integrate - This is the time the unit "looks" at an internal zero.
3. Reference Integrate - This is the time the unit "looks: at an internal reference voltage.
4. Calculate - This is time the firmware does some calculation on all the data it just measured.

Also, the :READ? does several things.
1. ABORT - Stops everything in the trigger model and goes back to idle.
2. :INIT - Starts the trigger model.
3. Fetch? - Obtains the reading.

Also, there are other parameters that affect reading rate.
FILTER- This is a digital filter. It takes a number of samples and calculates an average.
Trigger delay - this is a delay from the time the trigger is received till the A/D conversion starts.
Autozero - This switches all four phases on the A/D converter. Disabling AutoZero cuts out Zero and reference integrate. This action increases the data rate but the reading could drift over time. This is not recommended for stable measurements.
AutoRange - This takes time to "find" the right range. Use fixed range whenever possible.
Display - It takes time to update the display. It can be disabled.

I hope all this makes some sense.

luiz.andre
Posts: 2
Joined: February 26th, 2013, 5:41 am
Country: Brazil

Re: 2182A Integration Time

Post by luiz.andre » March 1st, 2013, 4:20 am

It makes a lot of sense! Thanks for the help!

The problem is that if I use NPLC 0.1, the noise measured is around 10^microvolts, and I need to measure nanovolt in the superconductor. With NPLC 1, I can measure just fine, but then I feel the time it takes to measure could be smaller. With a fixed range, when I turn the analog and digital filters off and disable autozero, the time drops about 5ms only, which doesn´t justify the lack of precision I get from disabling everything.

Is there any other way I can improve the time (with nplc still 1 because of the noise) or will I have to do with it the way it is?
Thanks very much for your good will!

Dale C
Keithley Applications
Keithley Applications
Posts: 2846
Joined: June 10th, 2010, 6:22 am
Country: United States
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Re: 2182A Integration Time

Post by Dale C » March 1st, 2013, 8:51 am

You have to experiment with each parameter to get the noise level and reading rate that is suitable for your application.

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